The present invention relates to a semiconductor memory device.
Conventionally, a number of functions of a dielectric including spontaneous polarization, high permittivity, electro-optic effect, piezoelectric effect, and pyroelectric effect have been applied to various devices. For example, pyroelectric effect of a dielectric is utilized for infrared linear ray sensors, piezoelectric effect is utilized for ultrasonic sensors, electro-optic effect is utilized for waveguiding optical modulators, and high permittivity is utilized for capacitors for DRAM (Dynamic Random Access Memory) and MMIC (Monolithic Microwave Integrated Circuit).
In recent years, a semiconductor memory is combined with a dielectric by using rapidly-developed thin film formation technique to develop FRAM (Ferroelectronic non-volatile memory) having high density and enabling high-speed operation. The non-volatile memory using a ferroelectric is high in write and read speed, low in operating voltage, and high in write and read tolerance, so that vigorous research and development is underway for its practical use not only as a substitute for a conventional non-volatile memory,but also as a memory capable of substituting for SRAM (Static Random Access Memory) and DRAM. A ferroelectric for use in such FRAM is required to be high in remanence, small in resistive electric field, low in leakage current, and high in tolerance for repeated polarization reversal.
As such ferroelectric, those using an oxide material with perovskite structure typified by PZT (Pb (Ti, Zr)O3: lead zirconate titanate) have been mainly used. However, there is generated such problem as increase of leakage current in a dielectric film and occurrence of a fatigue phenomenon in which spontaneous polarization is decreased by repetition of polarization reversal. Particularly, for substituting FRAM for DRAM, characteristics of the dielectric film of a capacitor is required to be unchanged with polarization reversal being performed 1015 times, by which the fatigue phenomenon becomes a serious problem in formation of DRAM and FRAM.
In recent years, a ferroelectric film with use of a compound having bismuth laminar structure is also being researched and developed. Specifically, a compound having bismuth laminar structure expressed by a composition formula of SrBi2 (TaxNb1-x)2O9 (O≦x≦1), i.e., SBTN, has a large polarization value and provides a square-shaped good polarization hysteresis against changes of electric fields, and decrease of dielectric polarization due to fatigue is scarcely seen. Accordingly, there has been proposed a semiconductor memory device having a capacitor using the SBTN (see Japanese Patent Laid-Open Publication No. HEI 8-23073). FIG. 5 is a schematic view showing the capacitor of the semiconductor memory device. The capacitor is composed of a lower electrode film 102, a dielectric film 103 consisting of SBTN, and an upper electrode film 104 formed in sequence on a ground layer 101. The upper electrode film 104 adheres to the dielectric film 103 by an anchoring effect generated by a portion of the upper electrode film 104 entering into recesses 105, 105 . . . formed by a grain portion on the surface of the dielectric film 103. It is noted that the recesses 105, 105 . . . on the dielectric film 103 are magnified in FIG. 5.
In the conventional semiconductor memory device, however, increase in composition percentage of Nb (niobium) among composition of SBTN forming a ferroelectric film of a capacitor increases a value of remanence of the ferroelectric film, and enlarges a diameter of grains composing the ferroelectric film, resulting in decrease in number of grain boundaries in the ferroelectric film. Consequently, increasing composition percentage of Nb among the SBTN for increasing capacity of the capacitor reduces a grain boundary portion of the dielectric film 103. This decreases the recess portion of the grain boundary of the dielectric film 103 into which the upper electrode film 104 enters, and thereby reduces adhesion of the dielectric film 103 and the upper electrode film 104. As a result, in heat treatment process and chemical treatment process after formation of the upper electrode film 104, exfoliation of the upper electrode film 104 occurs, which leads to decrease of a manufacturing yield of the semiconductor memory device.